1) Field of the Invention
The present invention relates to an optical transmission system. In particular, the present invention relates to an optical transmission system which performs WDM (wavelength division multiplex) transmission.
2) Description of the Related Art
The optical network technology is a core to develop a base of multimedia communication. It is desired that the service becomes available in a wider area and further sophisticated. In addition, since the transmission rates keep on increasing due to the recent explosive spread of the Internet and the like, the transmission capacities of the current systems in which optical signals of a single wavelength are transmitted in each optical fiber are insufficient. For example, it takes much time to transfer moving image data.
Therefore, WDM has been developed as a technique for efficiently utilizing existing optical fibers. The WDM is a technique in which signals in a plurality of channels are concurrently transmitted through a single optical fiber by multiplexing light having different wavelengths. A characteristic feature of the WDM optical amplification is the control in the ALC (automatic level control) and AGC (automatic gain control) modes.
In the ALC, the output level of each optical amplifier is maintained constant even when the input varies. Specifically, the gain of the optical amplifier is set so that the total output Pt (=Pn×n) of the optical amplifier is maintained constant regardless of the input level of the optical amplifier, where n is the number of the multiplexed wavelengths, Pn is a target output level at each wavelength, and the gain is a ratio of the optical output level to the optical input level.
In the AGC, the gain of the optical amplifier is maintained constant. Therefore, when the input level of the optical amplifier varies, the output level varies according to the variation of the input level. In the WDM transmission, smooth optical amplification control is realized by using the ALC and AGC in combination.
The optical amplifiers used in WDM nodes include optical preamplifiers and optical postamplifiers. The optical preamplifiers are multiwavelength optical reception amplifiers each of which receives and amplifies optical signals having multiple wavelengths and being transmitted from a node in the preceding stage, and the optical postamplifiers are multiwavelength optical transmission amplifiers each of which amplifies optical signals having multiple wavelengths and being internally processed for transmission to a node in the following stage.
Usually, the optical preamplifiers have both of the ALC and AGC functions, and the optical postamplifiers have only the AGC function. In addition, in the optical preamplifiers, the gain is initially set in an ALC mode at the time of initial setting, e.g., on a startup of a system or the amplifier, and thereafter the mode is changed to an AGC mode. Thus, the optical preamplifiers become operational. On the other hand, the optical postamplifiers are in an AGC mode in both of initial setting and operation, where the gain of each optical postamplifier is preset.
As described above, in the optical preamplifiers, the gain is initially set in the ALC mode according to an optical input level so as to maintain an optical output level constant, and thereafter the gain is maintained constant in the AGC mode. Therefore, when the optical input level of each optical preamplifier is not stable at the time of initial setting, the gain cannot be accurately set.
The ring network is widely used as a configuration of WDM networks. Hereinbelow, a problem which occurs in the case where an ALC mode is used in a WDM ring system is explained.
If an optical input level of an optical preamplifier in a node is not stable at the time of initial setting, the gain cannot be accurately set, and therefore amplification is performed from the unstable level. When optical signals are amplified in such a manner (i.e., optical signals are amplified, before the change to the AGC mode, in the stage of the ALC mode in which the gain is not accurately set) and transmitted to a next node, optical signals having unstable levels circulate through a plurality of nodes. Thus, the efficiency in system operation is decreased.
FIG. 11 is a diagram provided for explaining circulation of optical signals which have unstable levels. The nodes 301 to 304, which perform WDM transmission, are connected with optical fiber cables so as to form a ring structure. Each of the nodes 301 to 304 comprises an optical preamplifier 301a, . . . or 304a and an optical postamplifier 301b, . . . or 304b. In FIG. 11, other constituents such as optical switches are not shown.
At the time of initial setting, an optical signal added in the node 304 is amplified by the optical postamplifier 304b, and input into the optical preamplifier 301a in the node 301. When the optical preamplifier 301a operates in an ALC mode, the optical preamplifier 301a outputs an optical signal having an unstable level. When the optical signal output from the optical preamplifier 301a is input into the optical postamplifier 301b, the optical signal having an unstable level is transmitted from the node 301 to the node 302.
When an optical signal which has an unstable level is transmitted to a next stage before the gain is set in the optical preamplifier 301a, as explained above, it is impossible to accurately calculate the gain of the optical preamplifier 302a in the node 302, which receives the optical signal transmitted from the node 301.
When the above operations are repeated in the respective nodes, resultantly, optical signals having unstable levels circulate through the ring. Therefore, the optical input and output levels of the optical preamplifiers and the optical postamplifiers in the nodes become unstable, and oscillation occurs. Thus, the efficiency in operation decreases, and the service cannot be quickly started.